The present invention is directed to a variable gain amplifier that may be used in RF communication applications to obtain variable amplification of analog signals while maintaining low distortion. Specifically, a variable gain RF amplifier is described having means to control the amount by which the gain changes with respect to the control voltage, while maintaining a high linearity and gain insensitivity to temperature variations.
Differential amplifiers providing a linear gain for a range of input signals are known in the prior art. They are often used to compensate for gain variations due to temperature and process that occur in different sections of a transceiver chain such as in mixers, filters, etc. However, prior art differential amplifiers designs do not adequately provide a means for varying the gain in response to an external signal such that the gain varies linearly, in decibels, with the external signal over a range of RF frequencies. Most prior art variable gain amplifiers do not have a high degree of linearity between input signal and output signal amplitudes. Furthermore, prior art differential amplifier designs fail to adequately provide a gain that is invariant to changes in temperature and manufacturing process parameters, nor do they control the amount by which the gain varies in response to a control voltage to provide a linear variation (in dB) at RF frequencies.
The present invention is directed to a variable gain amplifier that may be used in RF telecommunication applications to obtain amplification of differential analog signals. It is also directed toward minimizing the effect that temperature variations have on the gain of the amplifier.
The variable gain RF amplifier, according to this invention, is comprised of a differential amplifier and a gain control circuit. The gain control circuit is provided by multiple transistors configured to affect a linear, variable resistor having a uniform voltage potential across each transistor which varies the feedback of differential amplifier. The variable resistor is connected between the differential amplifier transistors and varies the gain of the differential amplifier by controlling the resistance of the variable resistor. Slope compensation to a gain control signal optimizes the change in resistance with respect to changes in the gain control signal. dr
The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description that is to be read in conjunction with the accompanying drawings, in which:
FIG. 1 illustrates a preferred embodiment of the variable gain RF amplifier; and
FIG. 2 illustrates the variable gain of the circuit of FIG. 1 in response to a gain control signal.